The Carrera name originates from the famous Carrera Panamericana (Carrera = race in Spanish). Porsche gave this name to the most powerful 356 at first. They were like the GT3 of today.
In 1973, Porsche introduced the Carrera RS. Like the 356, it was the most powerful, race-ready model. After the first major restyling, the Carrera continued as the best model of the line-up. During this time, the 911 was offered with a wide choice of engines, with the Carrera on top.
The Turbo was introduced in 1975, outpowering the Carrera as the most powerful 911. Shortly after, the 911 SC was released as a single n/a model - the Carrera name was no longer in use (SC actually meant Super Carrera, but the car has always been officially called "SC").
Meanwhile, the Carrera name was used once again for a race-ready model, but it was a 924.
When the Carrera name was brought back to the 911 product line in 1984, it was still the only n/a model: the Carrera was now the base 911. This situation hasn't changed since then.
The 911 remains the correct name. There have been non-911 Carreras (the 356, the 924 and of course the Carrera GT). On the other hand, many 911 models are not Carreras: the Turbo, GT2 and GT3, and the Targa for instance.
You can find more info on the 911 on Wikipedia:
http://en.wikipedia.org/wiki/Porsche_911